Installation for treating an untreated flow by simple sedimentation after ballasting with fine sand

ABSTRACT

In a method of treating an untreated flow of liquid charged with particles and/or colloids, the flow is caused to flow in a coagulation zone that is kept turbulent and in which the flow is mixed with a coagulant. The untreated flow and a granular material which is insoluble in and more dense than the flow are caused to flow in an intermediate zone in which the granular material is maintained in suspension. The flow with substantially all of the added granular material and the colloids or particles aggregated therein is caused to flow in a sedimentation zone in which a clarified effluent is separated from sludge consisting of the granular material and the aggregated particles or colloids. The sedimentation zone has an undivided flow cross section with no separator plates and the effluent is caused to flow therein at an average settling speed of at least 15 m/h.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a method of and an installation forseparating solids and liquids (in particular, but not exclusively, forclarifying or purifying effluent) bycoagulation--flocculation--sedimentation without separator plates (i.e.simple sedimentation).

2. Description of the Prior Art

This invention is in the tradition of techniques for treating effluentby physical-chemical treatment (with formation of floc) followed bysedimentation (referred to as physical-chemical sedimentation forshort).

The first simple physical-chemical sedimentation devices allowedoperation at a "mirror" rate (i.e. the flowrate treated in m³ /h perunit surface area of the sedimentation device in m²) in the order of onemeter per hour; this upper limit was imposed by the lightness of thephysical-chemical floc and the low settling speed of the floc ineffluent.

French Patent 1,411,792 discloses a major improvement consisting in thesimultaneous addition to the effluent to be treated of auxiliary solidclarification agents (for example fine sand with a particle size from 20μm to 200 μm), a polymer and optionally other chemical productsroutinely used in purification of effluent (flocculation agents).

The floc that forms traps grains of sand which increase its density andtherefore its settling speed; the effluent charged with floc flows intoa sedimentation reactor from which it overflows (whence a decreasingfield of speed from the entry at the bottom to the exit at the top);settling speeds in the order of 6 m/h to 8 m/h are routinely achieved ininstallations using this principle (these installations are usuallycalled "CYCLOFLOC" installations).

The sand is recovered, for example by hydrocycloning of the sandysludge, and recycled to the entry of the installation.

French Patents 2,553,082 and 2,071,027 describe a technique in whichcontact between the untreated effluent and the granular material isbrought about by upward flow of the untreated effluent through afluidized bed of the granular material (sand, in practice), with anupward speed that decreases continuously, and with the concentrationregulated with the aid of a sample taken in the fluidized bed of a purgeof sand laden with sludge which is recycled after separating the sludge.Separator plates are provided to improve sedimentation above thefluidized bed.

The benefits of using inclined separator plates for treatment ofeffluent is discussed in many documents including U.S. Pat. Nos.4,142,970, 4,290,898 and 4,388,195.

"FLUORAPID" sedimentation units using the above principle can achievemirror speeds up to 8 m/h to 15 m/h.

Another sedimentation method is described in French Patent 2,552,082;this method does not use sand; instead a thickening and sedimentationintermediate chamber is provided between a reaction chamber(flocculation and/or precipitation) and a sedimentation chamber withseparator plates. The reaction chamber has two chambers in communicationat their top and bottom ends and an axial flow screw causing a strongflow between the two chambers. The reaction chamber also receives someof the sludge recovered from the bottom of the intermediate chamber.

The effluent charged with floc overflows into the upper part of theintermediate chamber in which the floc is thickened and in which 85% to90% of the floc is deposited on the bottom of the chamber. The partiallyclarified effluent then enters a sedimentation chamber with separatorplates for further sedimentation. It seems that speeds in the order of35 m/h can be obtained in this way if the constraints in terms oftreated effluent quality are moderate.

A more recent proposal is a treatment method also directed to increasingthe sedimentation outlet speed without compromising the quality of thetreated effluent and is described in U.S. Pat. No. 4,927,543; thismethod is implemented using installations usually called "ACTIFLO"installations.

In this method the effluent is injected into a stirred chamber formixing and destabilizing the colloids into which the granular material(fine sand, in practice) and the reagents are also fed. The mixture ofeffluent, sand, reagent and floc being formed is then passed into astirred intermediate aggregation chamber in which the floc that began toform in the mixing chamber increases in size without settling. Themixture of effluent and aggregated floc is then fed into a sedimentationchamber with separator plates. The mixture of sand and sludge recoveredfrom the bottom of the sedimentation chamber with separator plates ishydrocycloned and the sand is recycled to the mixing chamber.

This method produces effluent of excellent quality with mirror settlingspeeds up to around 100 meters per hour.

It is clear that the successive advances resulting from the techniquesmentioned above consist, for a given quality of treated effluent, in anincrease in the settling speed, firstly using ballasted floc(CYCLOFLOC), then by combining the use of sedimentation using separatorplates with floc ballasted with sand (FLUORAPID) or sludge that haspreviously settled, and finally by improving the quality of preparationof the floc through use of specific stirring conditions, again incombination with sedimentation using separator plates (ACTIFLO).

Thus all the recent methods described in the literature, including theACTIFLO method, are based on the use of separator plates forsedimentation wherever mirror settling speeds in excess of 15 m/h, forexample, are required.

These separator plates represent a non-negligible element of the cost ofthe installation, both through their inherent cost and through theresulting installation and cleaning constraints.

The present invention is therefore directed to improving the overalleconomy of sedimentation by eliminating the separator plates withoutcompromising the quality of the treated effluent.

In a manner that the person skilled in the art will find surprising andunexpected, we have discovered that careful physical-chemicalpreparation of the floc around a granular ballasting material of thetype proposed in U.S. Pat. No. 4,927,543 can yield high settling speedsdespite the absence of separator plates and therefore use a process thatis more economical and simpler than existing processes.

As explained below, the floc formed by the method of the inventionyields mirror speeds of as much as several tens of meters per hour,which is very much greater than the speeds of at most around ten metersper hour obtained with flocs produced using the CYCLOFLOC method,combined with treated effluent of very high quality.

SUMMARY OF THE INVENTION

The invention proposes a method of treating an untreated flow of liquidcharged with particles and/or colloids wherein:

the untreated flow is caused to flow in a first or coagulation zone thatis kept turbulent and in which the flow is mixed in controlledproportions with a coagulant reagent,

the coagulated flow and a granular material added in controlledproportions and which is insoluble in and more dense than the flow arecaused to flow in a second or intermediate zone in which turbulence ismaintained in order to maintain the granular material in suspensionwhile colloids or particles in the flow aggregate around particles ofthe granular material;

the flow with substantially all of the added granular material and thecolloids or particles aggregated therein is caused to flow in a third orsedimentation zone in which a clarified effluent is separated fromsludge consisting of the granular material and the aggregated colloids;and

the sludge is collected, the granular material is extracted from it torecycle it and the sludge is extracted without granular material;

wherein the sedimentation zone in which the clarified effluent and thesludge are separated has an undivided flow cross-section and the flowladen with granular material and with aggregated material is caused toflow therein at an average settling speed of at least 15 m/h.

The untreated flow is preferably effluent to be treated.

Thus the granular material is added in the intermediate zone at thelatest. It can be fed into the coagulation zone; it can be fed in atmore than one location (the coagulation zone and the intermediate zone).

The mean speed (mirror speed) concept mentioned above arises out of thefact that the flow of effluent to be treated fluctuates in practice.

In other words, sedimentation is effected freely without closely spacedsubdividing mechanical members such as separator plates in the path ofthe flow. Distribution members such as partitions or troughs can ofcourse be provided for reasons of hydraulic control, dependent on thegeometry of the sedimentation zone.

In accordance with preferred features of the invention, some of whichmay be combined with others:

the granular material is fine sand;

the mean particle size of the sand is between approximately 20 μm andapproximately 300 μm, preferably between 80 μm and 200 μm;

the settling speed is at least 35 m/h;

the speed gradient in the intermediate zone is between 70 s⁻¹ and 450s⁻¹, preferably between 150 s⁻¹ and 250 s⁻¹ ;

a flocculant agent is fed into the untreated flow in a first part of theintermediate zone or flocculation zone which is kept turbulent, theremainder of the intermediate zone constituting a maturation zone inwhich aggregation of the particles and colloids is completed;

further flocculant agent is fed into the maturation zone;

at least some of the granular material is fed into the flocculationzone;

the flocculation zone is stirred with a speed gradient betweenapproximately 100 s⁻¹ and approximately 450 s⁻¹, preferably between 200s⁻¹ and 250 s⁻¹, and more preferably in the order of approximately 200s⁻¹ ;

the speed gradient in the maturation zone is less than the speedgradient in the flocculation zone;

the speed gradient in the maturation zone is between 70 s⁻¹ and 300 s⁻¹,preferably between 150 s⁻¹ and 200 s⁻¹, and more preferably in the orderof approximately 150 s⁻¹ to 200 s⁻¹ ;

a cyclone or vortex is generated in the sedimentation zone by feedingthe flow from the second zone from a predetermined side of a verticalaxis of symmetry of the sedimentation zone; and

the granular material is separated from the sludge by hydrocycloning,screening or centrifuging.

The speed gradient G (s⁻¹) is expressed by the formula:

    G=(P/μ·V).sup.1/2

where

P=power dissipated in the liquid (Watts),

B=viscosity of the liquid (Pa.s), and

V=volume of the liquid (m³).

The existence of minima and maxima thresholds for the speed gradientvalues stated above is explained by the fact that the granular materialmust be kept in suspension, contact between small floc being formed withfloc already formed around particles of granular material must occur athigh relative speeds and shear stresses must be reduced to levels whichdo not cause any significant deterioration of the floc that is formed.

The invention also proposes an installation for treating an untreatedflow of liquid, including in series:

a first coagulation zone having a main entry channel connected to asource of untreated flow and a secondary channel connected to a sourceof coagulant reagent,

stirrer means in the coagulation zone,

a second or intermediate zone communicating with the first zone;

second stirrer means in the intermediate zone;

a secondary channel connected to a source of granular material that isinsoluble in and more dense than the untreated flow, discharging intothe coagulation zone or into the intermediate zone,

a third or sedimentation zone communicating with the second zone andhaving in its upper part an outlet channel for clarified effluent and inits lower part a sludge removal channel; and

a separation zone adapted to recover the granular material in the sludgeand having an outlet channel connected to the source of granularmaterial;

wherein the sedimentation zone has an undivided flow crosssection anddimensions, given a predetermined inlet flowrate value, such that theaverage mirror speed therein is at least 15 m/h.

The coagulation zone is in practice a tank but it must be understoodthat this zone can instead be formed by a portion of pipe into which thecoagulant reagent is injected, the stirring required for coagulationpossibly being generated by a bend in the pipe, a drop, a static mixeror any other device enabling the coagulant reagent to be brought quicklyinto contact with the entire flow.

In accordance with preferred features of the installation of theinvention, some of which may be combinable with others:

the sedimentation zone has dimensions such that the mirror speed thereinis at least 35 m/h;

the stirrer means of the intermediate zone are connected to meansadapted to generate by means of the stirrer means speed gradientsbetween 70 s⁻¹ and 450 s⁻¹, preferably between 150 s⁻¹ and 250 s⁻¹ ;

the intermediate zone includes a flocculation zone into which acoagulated flow inlet channel and an inlet secondary channel connectedto a source of flocculant agent discharge, followed by a maturationzone;

the maturation zone includes a flocculant agent inlet second channel;

the secondary inlet channel connected to the source of granular materialdischarges into the flocculation zone;

the second stirrer means in the intermediate zone are connected to meansadapted to generate by means of the second stirrer means speed gradientsin the flocculation zone between approximately 100 s⁻¹ and approximately450 s⁻¹, preferably between 200 s⁻¹ and 250 s⁻¹, and speed gradients inthe maturation zone between approximately 70 s⁻¹ and approximately 300s⁻¹, preferably between 150 s⁻¹ and 200 s⁻¹ ;

the sedimentation zone is a circular crosssection chamber

the maturation zone is a chamber at the center of the sedimentationzone;

the maturation zone:

i) is connected at its entry end to a downwardly extending tube;

ii) contains a wall in front of the outlet of the tube and adapted to becrossed by underflow; and

iii) is surrounded by an upwardly projecting tubular wall, the outletchannel for clarified effluent extending from the upper zone of thesedimentation zone to the exterior of the tubular wall;

the maturation zone communicates with the sedimentation zone via a tubedischarging into the sedimentation zone away from a vertical axis ofsymmetry of the latter, so as to generate a cyclone or vortex;

the sedimentation zone is circular and the tube discharges thereintangentially;

the sedimentation zone has in a central position a tubular wallcommunicating with a clarified effluent removal trough, the tubular wallcan have no bottom and can be bordered in its upper part by a guttercommunicating with the trough; the wall can also have a bottom andcommunicate directly with the removal trough, for example via thebottom.

One installation constructed according to this principle has beenoperated with mirror settling speeds in the order of 50 m/h and possiblyup to 100 m/h and above.

There was previously nothing to suggest the possibility of obtainingsuch settling speeds without using a sedimentation plant includingseparator plates.

To the contrary, using Stokes formula to evaluate the settling speed ofspherical particles of sand (density 2.65 T/m³) in effluent gives speedsin the order of 30 m/h for particles 100 μm in diameter or 8 m/h forparticles 50 μm in diameter.

As already emphasized above, obtaining high settling speeds (say greaterthan 15 m/h) has systematically been attempted until now by combiningthe use of separator plates with a particular way of preparing the floc.

It is nevertheless apparent that careful preparation of the floc aroundthe granular material has a synergistic action when combined with theinherent settling speed of the pure granular material, and that theincrease in diameter resulting from flocculation has a positive effecton the settling speed which, by virtue of the compactness of the flocresulting from its particular mode of preparation, is greater than thenegative action of the reduction in the density of the floc compared tothat of the pure granular material.

Objects, features and advantages of the invention will emerge from thefollowing description given by way of non-limiting example withreference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic elevational view of a first effluent treatmentinstallation of the invention;

FIG. 2 is a diagrammatic elevational view of a second effluent treatmentinstallation of the invention;

FIG. 3 is a diagrammatic plan view of part of the second installation;

FIG. 4 is a diagrammatic elevational view of a third effluent treatmentinstallation of the invention;

FIG. 5 is a diagrammatic plan view of the third installation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagrammatic representation of a effluent treatmentinstallation 10 embodying in series:

a first chamber or coagulation zone 11A having a main inlet channel 12connected to a source of effluent to be treated and a secondary channel13 connected to a source of coagulant reagent 13A;

at least one stirrer member 16 in this coagulation zone;

a second or intermediate zone communicating with the first zone, here bymeans of an underflow (in an alternative embodiment, not shown, thiscommunication could of course be by means of an overflow), and connectedby respective secondary channels 14 and 15 to a source 14A of flocculantagent and to a source 15A of a granular material that is insoluble ineffluent and is more dense than effluent;

at least second stirrer means in this intermediate zone, connected tomeans adapted to generate in all of this zone speed gradients in theorder of at least approximately 70 s⁻¹ to approximately 450 s⁻¹,preferably in the order of approximately 150 s⁻¹ to approximately 250s⁻¹ ;

a third or sedimentation zone 21 communicating with the second zone(here by means of an overflow) and having in its upper part an outletchannel 22 for clarified effluent and in its lower part a sludge removalchannel 23, here provided with a pumping unit 24;

a separation zone constituting here the source 15A of granular material,adapted to recover the granular material in the sludge and having asludge outlet channel 15B and a granular material outlet channel 15C.Separation can be obtained by any appropriate known means available tothe person skilled in the art for separating the recycled sand from thesludge, including hydrocycloning, screening and centrifuging.

In this example the sedimentation zone or chamber has a circular crosssection (it can be any other shape, with hoppers at the bottom forremoving the sludge) and advantageously has a scraper 25 running alongthe conical bottom of this zone and rotated by a motor 26, usually at alow speed so that in practice its peripheral speed is less than or equalto about 10 cm/s.

A trough 27 for recovering clarified effluent is provided in the upperpart of the sedimentation chamber.

In accordance with the invention the sedimentation zone has an undividedflow cross section, i.e. it does not feature any mechanical membersdividing the flow on approaching the clarified effluent outlet channel.Thus there are no separator plates in the sedimentation zone.Distribution members such as partitions or troughs can of course beprovided for reasons of hydraulic control, depending on the geometry ofthe sedimentation zone.

However, the sedimentation chamber is dimensioned to suit a nominalflowrate Q of effluent to be treated such that the "mirror speed" Vm inthis chamber is at least 15 m/h, and preferably at least 35 m/h.

In other words, the sedimentation chamber has a horizontal cross sectionS such that:

    Q/S>15 m/h

The coagulant is of any appropriate known type (for example ferricchloride or aluminum sulfate).

The intermediate zone is formed in this example by two chambers inseries: a flocculation chamber 11B into which the second flocculantagent entry channel 14 and the granular material entry secondary channel15 discharge and a maturation zone 18.

In an alternative embodiment, not shown, the maturation zone includes asecondary channel connected to the source 14A for further addition offlocculant agent.

The flocculant agent is of any appropriate known type (for example theanionic and/or cationic polyelectrolyte type).

In an alternative embodiment, not shown, the granular material entrysecondary channel 15 discharges into the first chamber 11A, or even intothe discharge and maturation chamber 18, or even into more than one ofthe chambers 11A, 11B and 18. It is preferable for the granular materialto be fed into the first chamber 11A and/or into the flocculationchamber 11B.

In this example the chambers 11A and 11B are of similar size, within abattery 11.

Each of the flocculation chamber 11B and the discharge and maturation 18has a stirrer, the speed gradient being in principle maximum in theflocculation zone. The flocculation chamber 11B has a stirrer 17 drivenby a motor, not shown, at a speed adapted to generate a speed gradientbetween 100 s⁻¹ and 450 s⁻¹ preferably between 200 s⁻¹ and 250 s⁻¹, andthe discharge and maturation chamber 18 includes a stirrer 19 driven bya motor 20 at a speed adapted to generate a speed gradient between 70s⁻¹ and 300 s⁻¹, preferably between 150 s⁻¹ and 200 s⁻¹.

The granular material is preferably sand (easy to obtain at low cost)with a mean particle size between about 20 μm and 300 μm, preferablybetween 80 μm and 200 μm.

FIGS. 2 and 3 show a different embodiment of the invention. They show aninstallation 30 in which elements analogous to those of FIG. 1 have thesame reference numbers increased by 20.

The main difference compared to FIG. 1 is that, although the coagulationand flocculation zones are outside the zone 41, as previously, thesedimentation maturation zone 38 is at the center of the sedimentationzone 41, communicating with the flocculation zone 31B via a downwardlyextending tube (or trough) 45'; a partition 46' facing the outlet fromthis tube is adapted to cause downward flow of the fluid mass as far asthe bottom of the maturation chamber 38.

The fluid mass leaves the intermediate chamber via an overflow.

To prevent any of the fluid mass from escaping directly to the outletchannel 42 a tubular partition 47' is provided around the intermediatechamber, its upper edge being higher than the normal level of the fluidmass in the sedimentation chamber.

Because of the steep inclination (typically exceeding 55°) of thebottom, the conical bottom of the sedimentation chamber has no scraperin this embodiment.

Compared to that of FIG. 1, the configuration of FIGS. 2 and 3 has theadvantage of being more compact when added to an existing installationand of enabling better distribution in the sedimentation zone.

FIGS. 4 and 5 show another embodiment of the invention. They show aninstallation 50 in which elements analogous to those of FIG. 1 have thesame reference numbers increased by 40.

The main difference compared to FIG. 1 is that the outlet from thematuration chamber 58 is a tube 65 which discharges laterally into thesedimentation chamber 61, away from and transversely to its verticalaxis of symmetry. To be more precise, the tube 65 advantageouslydischarges tangentially to the lateral wall of the sedimentation chamber61, producing a cyclone or vortex which helps to acceleratesedimentation.

At the center of the chamber is a bottomless tubular wall 66 over thetop of which overflows clarified effluent which is collected by anannular gutter 67 connected to a take-off trough 68. As an alternative,the tubular wall 66 can have a bottom (as symbolized by the chain-dottedline 70) hydraulically isolating the interior of this wall from theremainder of the sedimentation unit, and the clarified effluentoverflows this wall before it is removed via the trough 62 which in thisexample runs direct from the inside of this wall.

The presence of two successive chambers in the intermediate zone ofFIGS. 1 through 5 is explained by the fact that in the case of effluentcharged with colloidal organic material it has been found that apreferred method of forming the floc to obtain settled effluent ofbetter quality is to feed the coagulant agent intended to destabilizethe colloidal solution (usually a high valency metal salt or a cationicpolymer) at the start of the preparation process, mixed with theuntreated effluent by stirring or by static mixing. After this firstdestabilizing mixing the flocculant polyelectrolyte is injected into thestirred flocculation zone with a relatively high speed gradient (between100 s⁻¹ and 450 s⁻¹, preferably around 200 s⁻¹), enabling the microflocresulting from coagulation to gather and to trap particles of granularmaterial held in suspension.

Finally, the mixture of effluent and floc is fed into the maturationchamber, stirred with a speed gradient of approximately 70 s⁻¹ toapproximately 300 s⁻¹, preferably in the order of approximately 150 s⁻¹.In the maturation chamber the floc aggregates to an optimum size betweenabout 0.5 mm and about 2.5 mm, depending on the quality of the incomingeffluent. The time it remains in the coagulation zone can be very short,depending mainly on the quality of the mixing that takes place in thiszone (this time is usually between a few seconds and three minutes oreven more (up to 10 minutes), preferably from 30 seconds to 1.5 minutesat the peak flowrate).

The time in the flocculation zone is advantageously between 0.5 minutesand four minutes, or even more (up to 10 minutes), preferably 1.5minutes to two minutes at the peak flowrate); the time spent in thematuration zone is between two minutes and eight minutes, or even more,up to 18 minutes (preferably three minutes to six minutes at the peakflowrate to be treated).

It will be understood that dispensing with separator plates in thesedimentation zone eliminates the installation constraints associatedwith the separator plates. For example, a round shape can be chosen forthe sedimentation chambers.

The invention combines preparation in a series of at least two chambers(flocculation and maturation, if the coagulation is carried out in-linebeforehand), and preferably in three chambers (coagulation,flocculation, maturation as in the above examples), the tanks beingrectangular or cylindrical, with simple sedimentation in a rectangularor cylindrical tank, depending on installation or civil engineeringconstraints.

The embodiment without separator plates of FIGS. 4 and 5 combinesphysical-chemical preparation of the ballasted floc described above withenhanced sedimentation in a cyclone or vortex type sedimentation unit.

This type of sedimentation unit has been tested in natural sedimentationand, without using separator plates, increases the mirror speeds thatcan be achieved: its use in combination with careful physical-chemicalpreparation of a dense ballasted floc as described above improvessedimentation performance compared to simple sedimentation.

A pilot installation has been constructed with the following parameters:

flocculation tank: 4 m³ with vigorous stirring (˜50 W/m³, i.e. a speedgradient of around 220 s⁻¹) into which ferric chloride is fed,

injection tank: 4 m³, also with strong stirring (˜50 W/m³, i.e. a speedgradient of around 220 s⁻¹) into which an anionic polymer and sand witha grain diameter of 130 μm from the underflow of a hydrocycloneseparating the extracted sludge from the recirculating sand are fed;

maturation tank: 15 m³ stirred more slowly (˜35 W/m³, i.e. a speedgradient of around 190 s⁻¹);

sedimentation tank: rectangular cross section 1.94 m×1.1 m=2.1 m²(mirror surface), height 1.5 m, front fed at the bottom and equippedwith a pyramid-shape hopper with flushed sludge and sand removal;

treated effluent recovered from a central trough 0.5 m wide.

The table below summarizes the results of two tests:

    ______________________________________                                                  TEST 1      TEST 2                                                  ______________________________________                                        Flowrate    100 m.sup.3 /h                                                                              200 m.sup.3/h                                       Ferric chloride                                                                           60 mg/l (pure product)                                                                      60 mg/l (pure product)                              Coagulation time                                                                          2.6 minutes   1.3 minutes                                         Polymer     0.9 mg/l      0.9 mg/l                                            Sand        8 kg/m.sup.3  4.6 kg/m.sup.3                                      Feed time   2.6 minutes   1.3 minutes                                         Maturation time                                                                           9 minutes     4.5 minutes                                         Mirror speed                                                                              45 m/h        90 m/h                                              Sludge removal rate                                                                       10%           5%                                                  Inlet MIS   255 mg/l      114 mg/l                                            Outlet MIS  31 mg/l       30 mg/l                                             Yield       87.8%         73.7%%                                              ______________________________________                                    

Compared to test 1, test 2 represents a "high" flowrate and a "low"concentration of materials in suspension (MIS).

The table shows the excellent efficiency in reducing MIS with highmirror speeds (45 m/h and 90 m/h).

It goes without saying that the foregoing description has been given byway of non-limiting example only and that numerous variants can be putforward by the person skilled in the art without departing from thescope of the invention. For example, the invention can be generalized tothe treatment of any other liquid charged with particles or colloids,for example baths for surface treatment of metals in the iron and steelindustry.

There is claimed:
 1. A system for clarifying an influent liquid througha sedimentation process comprising:a) a coagulation zone for mixing acoagulation reagent with the influent liquid; b) an intermediate zonedisposed downstream of the coagulation zone for receiving mixed liquidfrom said coagulation zone and mixing therein an insoluble granularmaterial having a density greater than the liquid so as to promoteflocculation, said granular material being added at least as early asthe intermediate zone; c) a sedimentation tank, having a wall structure,disposed downstream of the intermediate zone and communicativelyconnected to said intermediate zone for receiving mixed liquidtherefrom, said sedimentation tank including an effluent outlet formedin an upper portion thereof for directing clarified effluent from thesedimentation tank; d) a separation zone for receiving combined granularmaterial and sludge from said sedimentation tank and for separating thegranular material from the sludge such that the granular material can berecycled through the system; e) a flow directing baffle, having a lowerterminal edge, interposed between said intermediate zone and saideffluent outlet, said baffle defining a downflow zone between saidintermediate zone and said baffle and an upflow zone on the side of thebaffle opposite said downflow zone; and f) wherein the baffle isparticularly spaced relative to the adjacent sedimentation tank wallstructure so as to permit unrestricted flow from the downflow zone tothe upflow zone.
 2. The system of claim 1 wherein at least a portion ofthe intermediate zone is disposed centrally within the sedimentationtank and includes a chamber having a surrounding wall structure andwherein the flow directing baffle is disposed in general parallelrelationship to the outer wall structure of the chamber.
 3. The systemof claim 1 wherein the sedimentation tank is formed without separatorplates and maintains an average mirror rate of at least 15 m/h whileproducing clarified effluent having at least a 60% reduction inmaterials in suspension relative to the influent.
 4. The system of claim1 wherein the sedimentation tank maintains an average mirror rate of atleast 35 m/h while producing clarified effluent having at least a 65%reduction in materials in suspension relative to the influent.
 5. Thesystem of claim 1 wherein the sedimentation tank maintains an averagemirror rate of at least 90 m/h while producing clarified effluent havingat least a 70% reduction in materials in suspension relative to theinfluent.
 6. system of claim 3 wherein the intermediate zone includes astirrer for producing speed gradients in the intermediate zone in therange of 70 s⁻¹ to 450 s⁻¹.
 7. The system of claim 3 wherein theintermediate zone includes a stirrer for producing speed gradients inthe intermediate zone in the range of 450 s⁻¹ to 250 s⁻¹.
 8. The systemof claim 3 wherein the sedimentation zone comprises a circularcross-section chamber.
 9. The system of claim 3 wherein the intermediatezone includes two separate chambers, a flocculation chamber having aninlet for a flocculation agent and a maturization chamber disposeddownstream from the flocculation chamber.
 10. The system of claim 9wherein the flocculation chamber includes an inlet for directing thegranular material into the flocculation chamber.
 11. The system of claim9 wherein the flocculation chamber includes a stirrer that generatesspeed gradients within the flocculation chamber in the range of 100 s⁻¹to 450 s⁻¹ ; and wherein the maturization chamber includes a stirrerthat generates speed gradients in the range of 70 s⁻¹ to 300 s⁻¹. 12.The system of claim 9 wherein the maturization chamber is disposedgenerally in the center of the sedimentation tank.